Currently, large quantities of liquids are transported in horizontal cylindrical tanks which are located within a frame which has the dimensions of standard ISO containers, so that, as a result, they can be transported and handled by standard container handling and transport systems. They may be transported by rail, road and sea and emptied by connecting a hose to a discharge pipe located at the bottom of the tank.
Bulk powders can also be transported in this type of tank, but, being bulk powders, they cannot be emptied in the same way. One method of emptying involves tipping the tank to an approximately 45° angle and then pressurising the tank to discharge the contents from one end through a hose. An alternative method involves a fluidising system covering a large proportion of the internal area of the base of the tank. Along the length of the tank there are arranged a plurality of discharge points through which the material is pneumatically conveyed after pressurisation of the tank. With such an arrangement the tank can be emptied while it remains horizontal.
An alternative method of fluidising the tank involves the provision of a plurality of conical outlets in a line along the length of the bottom half of the tank. The material can flow into these outlets by gravity and thence be pneumatically conveyed to storage. Flow is by gravity towards the outlets with only the bottom of the conical section being aerated. The provision of the conical outlets reduces the total storage space. Furthermore the overall shape, including the conical outlets is not ideal for a pressure vessel.
The use of an internal fluidising membrane, which covers a large proportion of the base area of the tank, enables the maximum bulk powder volume to be transported. Due to the conventional methods of internal membrane support structure which have been employed, this method results in a considerable increase in the weight of the overall container. The amount of bulk material which can be transported is limited by the maximum transportable weight of the container together with its contents, typically between 30 and 34 tonnes. It is therefore desirable to utilise a fluidisation system which leaves maximum internal volume for bulk material and also adds as little as possible to the weight of the container.
To enable the fluidised material to flow by gravity towards the discharge pipes, the fluidising membrane is sloped at an angle to the horizontal which is typically between 5 and 10°. Previous methods for supporting the fluidising membrane include the use of internal frames onto which flat panels of fluidising membrane are bolted. These frames cause a reduction in volume due to their height inside the tank. They are usually constructed in flat panel format which creates a number of areas on the membrane which do not fluidise well at the junctions between the panels when fixing bolts are present. This results in a number of places where the material does not flow, thereby resulting in relatively large amounts of residual materials in the tank which cannot be discharged. Furthermore, the flat panels require significant reinforcement to take the weight of bulk powder such as cement over such a large flat area. This reinforcement is expensive and adds weight to the tank which affects the economics of its use due to the overall transport weight restrictions for such containers.